Plant organ system: translate information between graphical and numerical form

The idea

This skill means moving confidently between a graph and the numbers it represents. In the plant organ system topic you might be given a graph of transpiration rate against light intensity, temperature, humidity or wind speed, and asked to read values, calculate a rate, or plot points from a table. To translate graphical to numerical form, read the axes and units, locate the required point, and state the value with its unit. To translate numerical to graphical form, choose a sensible scale, plot each pair of values accurately, and draw a line or curve of best fit. You also need to describe patterns: for example, as light intensity rises, transpiration rate rises until stomata are fully open, then levels off. Accuracy in reading scales and labelling axes is assessed through practical and data-handling questions.

What earns the marks

• Read values from a graph accurately by identifying the scale of each axis, including any multiplier such as 'rate in cm³ per minute ×10'.
• Convert tabulated numerical data into a plotted graph by choosing equal intervals, labelling both axes with quantity and unit, and plotting points to within half a small square.
• Calculate a rate from numerical data using rate = change in mass ÷ time, or read a rate directly from a graph and quote its unit.
• Describe the relationship shown, for example 'as temperature increases from 10 °C to 30 °C, transpiration rate increases, then changes little above 30 °C'.
• Use interpolation and simple extrapolation carefully, recognising that extrapolated values are predictions beyond the measured range.

Common mistakes and how to put them right

• Misreading the scale, such as treating every gridline as 1 when each represents 5.

  Put it right by counting the intervals between labelled values before reading any point.

• Omitting units or writing the wrong unit, such as giving a rate as '8' instead of '8 cm³ per minute'.

  Put it right by copying the unit from the axis label.

• Joining plotted points with straight lines when the relationship is a smooth curve.

  Put it right by drawing a single line or curve of best fit that shows the overall trend.

• Reversing axes when plotting, so the independent variable is on the y-axis.

  Put it right by placing the variable you changed on the x-axis and the measured result on the y-axis.

Exam tips

• Before reading a graph, state the quantity and unit on each axis to yourself; this prevents scale and unit errors.
• Show your working when calculating a rate, including the values substituted, so method marks can be awarded even if the final number is wrong.
• When describing a trend, quote at least two pairs of values from the graph and use the phrase 'as ... increases, ... increases' to make the relationship explicit.